Removed text/plain from MIME types in gedit/scala.lang.

text/plain shouldn't appear in this file, since that causes
gedit to apply Scala highlighting to plain text files.
I also updated the install path in the comment at the top of the
file.
Merge pull request #82 from adam-yorba/master

please pull: updated syntax highlighting file for gedit
Merge branch '2.10.x'
Merge pull request #85 from adriaanm/2.10.x

2.10.x