Upgraded build pack utils.
Set defaults for some common env variables.
Shell substitution doesn't work when setting environment variables (i.e. PATH=/Users/danielmikusa/.pyenv/shims:/Users/danielmikusa/.pyenv/bin:/Users/danielmikusa/.rbenv/shims:/Users/danielmikusa/.rbenv/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/opt/X11/bin:/Users/danielmikusa/Development/tools/apache-ant-1.9.3/bin:/Users/danielmikusa/Development/tools/apache-maven-3.1.1/bin:/Users/danielmikusa/Development/languages/groovy-2.1.6/bin:/Users/danielmikusa/Development/frameworks/grails-2.3.0/bin:/Applications/Xcode.app/Contents/Developer/usr/libexec/git-core/:/Users/danielmikusa/Development/languages/node-v0.10.0-darwin-x64/bin:/Users/danielmikusa/Development/tools/gradle-1.9/bin:/Users/danielmikusa/Development/tools/spring-0.5.0.M1/bin:/Users/danielmikusa/Development/languages/go-1.2/bin:/Users/danielmikusa/Code/Go/bin:. doesn't work).  To work around that, bp is rewriting the env file.  This allows something like PATH=@PATH:., where @PATH is rewritten to the value of PATH at the time of the rewrite.  In this case, it's runtime which is generally what you'd want.