Adding missing dependency to build file

If one does not have this dependency in global sbt settings, sbt fails to load with strange error:
```
C:\Users\Jerzy\Workspaces\GitHub\scala-ide-docs>sbt
(...)
[error] java.lang.NoClassDefFoundError: org/eclipse/jgit/storage/file/FileRepository
[error] Use 'last' for the full log.
Project loading failed: (r)etry, (q)uit, (l)ast, or (i)gnore? r
```