Increased diamond version in requirements.txt

This is to fix the error
"No distributions at all found for python-configobj"
when using lower version of diamond.

Corresponding diamond issue:
https://github.com/BrightcoveOS/Diamond/issues/462

Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>
Merge pull request #168 from kmkanagaraj/fix-requirements

Increased diamond version in requirements.txt

Reviewed-by: John Spray <john.spray@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>