Bump version to 1.9.0 

Merge pull request #976 from libgit2/cmn/update-17 Update libgit2 to v1.7.2

Merge pull request #975 from libgit2/cmn/update-16 Update to libgit2 v1.6.5

Merge pull request #964 from libgit2/cmn/bump-17 Update to 1.7.1

Merge pull request #957 from libgit2/cmn/bump-1.6.3 Update to 1.6.3

Merge pull request #953 from libgit2/cmn/v1.6 Create and update v1.6 branch

Merge pull request #945 from libgit2/cmn/bump-libgit2-15 Update to v1.5.1

Merge pull request #944 from libgit2/cmn/bump-libgit2-14 Update to v1.4.5

Bump versino to 1.5.0.1 

Merge pull request #931 from libgit2/cmn/bump-version-15 Bump rugged version to v1.5.0