Update .travis.yml with correct libbison path.
Update .travis.yml with correct libbison path.

- Closes #16

 On branch master
 Your branch is up-to-date with 'origin/master'.

 Changes to be committed:
	modified:   .travis.yml

ben-marshall/verilog-parser,ben-marshall/verilog-parser,ben-marshall/verilog-parser