Do not fail .bashrc if travis is not installed

[lifeofguenter]

Currently installing travis (bash completion) might add something like this into your .bashrc:

# added by travis gem
[ -f /Users/guenter/.travis/travis.sh ] && source /Users/guenter/.travis/travis.sh

This will unfortunately cause bash to spawn with a return-code of 1.

My fix will solve this issue and make sure that it will work if those files exist, and that it won't throw an error should they not exist:

    [ ! -s ~/.bashrc ] || echo yes; echo $? # file exists, outputs "yes", rc=0
yes
0
    [ ! -s ~/.bashrrc ] || echo yes; echo $? # file does not exist, outputs nothing, rc=0
0
    [ -s ~/.bashrc ] && echo yes; echo $? # file exists, outputs "yes", rc=0
yes
0
    [ -s ~/.bashrrc ] && echo yes; echo $? # file does not exist, outputs nothing, rc=1
1