exporting PATH and LIBRARY_PATH automatically

inspired by https://github.com/shunjikonishi/heroku-buildpack-ffmpeg/blob/master/bin/compile
Merge pull request #1 from jigfox/patch-1

exporting PATH and LIBRARY_PATH automatically
remove note about environment

the variables are now set automatically
Merge branch 'master' of https://github.com/jigfox/heroku-buildpack-phantomjs into jigfox-master
Merge branch 'jigfox-master' into jigfox-pr