oo-exec-ruby: Unset X_SCLS before scl enable

Modify oo-exec-ruby to delete any occurrence of 'ruby193 ' from the
X_SCLS environment variable before running scl enable ruby193.

scl enable sets the X_SCLS environment variable to keep track of what
collections have been enabled in the current environment.  If a collection
is already enabled, scl enable does not re-enable it (this behaviour is by
design; see bug 1016962).  This commit forces the scl enable invocation in
oo-exec-ruby to set up the ruby193 environment even in nested invocations
so that oo-ruby works properly even if it is run in an environment that was
set up by an earlier scl enable invocation (e.g., from an earlier oo-ruby
invocation) in case that environment was modified (e.g. PATH was munged)
between the two scl enable invocations.

This commit fixes bug 1017248.
CartridgeRepository#install: Don't keep context

Don't preserve the context when installing a cartridge into the cartridge
repository.  The cartridge needs to be relabeled using the default
contexts.

This commit fixes bug 999117.