Allow Leiningen standalone jar to be removed from slug.

It should download on-demand for when it's needed (interactively via
heroku run, etc) but is no longer required.
Set LEIN_BUILD_TASK if lein isn't in Procfile.
Document uberjar-based deploys in readme.
Bash typo.
Add some notes for uberjar builds.
Fix file.encoding property in .profile.d.
Mention using $JVM_OPTS in the Procfile when using an uberjar.
More details on Procfile's java -cp vs java -jar.
Bump Leiningen to 2.2.0.
Move offline out of the production profile.
No need for m2 to live in the slug when uberjarring.
Merge branch 'master' into uberjar
Emit a warning if Leiningen version is not specified.
Don't need to mention mirrors in the readme.
Re-word uberjar instructions, remove redundant part.
Don't use uberjar as an example in the hacking section.
Use :uberjar-name as the heuristic instead of grepping the Procfile.

Grepping the Procfile would result in false positives if people had
lein invocations inside a bin/run script or something like that.
Update readme to put uberjar section first.
Mention that certain some features must be manually invoked with uberjars.
Use $BUILD_DIR consistently.