Maven central release setup

[pietrygamat]

Updates the project to conform with Maven Central requirements

Edit: See also #23 (comment)

[pietrygamat]

This PR builds upon #66, and I'd rebase this one if the other is accepted. Opening for the purpose of review.
Addresses some points mentioned in #23

[tresf]

My main concern with this PR is whether or not the native libraries will still be placed in the right location. It'd also be nice to see Travis-CI in a better state (to no fault of yours) before merging.

For starters, Travis bumped the build environment (14.04 -> 16.04), so cmake3 is now simply cmake. macOS seems to be failing with the following, which I haven't had time to research.

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0:compile (default-compile) on project jssc: Fatal error compiling: error: release version 6 not supported -> [Help 1]

[pietrygamat]

Looks like Travis is building with JDK12+ while the compilation target of 1.6 is only supported up to JDK11(LTS).
FYI, This PR was tested with Corretto JDK8 and Corretto JDK11, but got the same error on 12.

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0:compile (default-compile) on project jssc: Fatal error compiling: error: release version 6 not supported -> [Help 1]

[pietrygamat]

My main concern with this PR is whether or not the native libraries will still be placed in the right location.

mvn clean install on root directory causes jssc.dll to be updated under src/main/resources-precompiled/natives/windows_64 on my Windows 10 using Visual Studio's CMAKE, so I assume nothing is broken (although more detailed setup instructions for windows would be welcome :) ). Ideally it should be created under target, but this is outside of scope for change.

[tresf]

Looks like Travis is building with JDK12+ while the compilation target of 1.6 is only supported up to JDK11(LTS).
FYI, This PR was tested with Corretto JDK8 and Corretto JDK11, but got the same error on 12.

Fixed via d0c6223.

[tresf]

Ideally it should be created under target, but this is outside of scope for change.

This was done intentionally via #55 although I'm open to ideas.

although more detailed setup instructions for windows would be welcome

Between Travis, Maven and CMake, the build environment is quite hard for even me to follow, especially when it's only touched on occasion. From what I recall correctly, the system was only written with developers (not releases) in mind and assumed a standard Maven setup, CMake installed from the website and some -- any version -- of MSVC available. I'm sure this can be improved and as I begin working on arm64/aarch64, I'll be setting this environment up again. Any specific questions are welcome.