fix($protractor): addMockModule and removeMockModule bugs identified in issue 764.

Calling addMockModule twice with the same module name added both modules and submitted each to
the browser for registration.  After this fix, calling addMockModule with the same module name
twice removes the first module from the list to be submitted and adds the new module to the end
of the list.

Calling removeMockModule with a name that was never added would incorrectly remove the most
recent module added with addMockModule.  After this fix, a similar call results in a no-op.

Closes #764.

Breaks $protractor.addMockModule, insofar as any code was writen that intentionally or
inadvertently depends on the old behavior.