feat: Add generic CI support

[Alorel]

Closes #245

This adds support for a generic CI provider configured entirely by environment variables.

Test repo and PRs: Alorel/test-generic-lockfile#2, Alorel/test-generic-lockfile#3

Usage (copied from README):

A generic CI setup allows greenkeeper-lockfile to function in any CI by getting all the data from environment variables.
All the environment variables are required; boolean values should be provided as the string "true" or "false".

Variable	Description	Type	Example
GK_LOCK_GENERIC_CI	Set this to true to trigger generic CI	boolean	true
GK_LOCK_GENERIC_CI_REPO_SLUG	Your GitHub repo slug	string	greenkeeper/greenkeeper-lockfile
GK_LOCK_GENERIC_CI_BRANCH_NAME	Name of the current branch	string	greenkeeper/lodash-4.0.0
GK_LOCK_GENERIC_CI_FIRST_PUSH	Whether this is the first push to the branch or not	boolean	true
GK_LOCK_GENERIC_CI_CORRECT_BUILD	Is this a regular build (not a pull request for example)	boolean	false
GK_LOCK_GENERIC_CI_UPLOAD_BUILD	Should the lockfile be uploaded from this build (relevant for testing multiple node versions)	boolean	true