Update CLI install to prefer local snapshots to those in a remote repo

Previously, InstallCommand used a custom Grape root and then walked
the tree of files downloaded by Aether to determine which files it
should install or uninstall. In some scenarios two files for the
same module would be present: one named foo-1.0.0.BUILD-SNAPSHOT.jar
and one named foo-1.0.0.BUILD-20140905.091809-2.jar. The former is
from the local repository and the later is from a remote repository.
In this case, the visitor would do the wrong thing and the latter
would be installed into lib.

This commit updates InstallCommand to determine the jars that it
should process by consulting GroovyCompiler's classpath, rather than
by walking Aether's cache. This approach selects the correct jar as
Aether has already figured this out as part of resolving the
dependency. It also brings InstallCommand into line with JarCommand.

The previous implementation used Java 7-specific File APIs. As part
of the above-described change this usage has been removed. The
install command can now be used on Java 6.

Fixes gh-1515