Fix subtree spec

subtree spec is currently failing on master:

```
  1) git::subtree when git version > 1.7.0 and < 1.7.11 should contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core] with creates => "/usr/src/git-subtree/git-subtree" and cwd => "/usr/src/git-subtree"
     Failure/Error: it { should create_exec('/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core').with({
       expected that the catalogue would contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core]
     # ./spec/classes/git_subtree_spec.rb:28:in `block (3 levels) in <top (required)>'

  2) git::subtree when git version > 1.7.0 and < 1.7.11 should contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core install] with creates => "/usr/lib/git-core/git-subtree" and cwd => "/usr/src/git-subtree"
     Failure/Error: it { should create_exec('/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core install').with({
       expected that the catalogue would contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core install] with creates set to "/usr/lib/git-core/git-subtree" but it is set to nil
     # ./spec/classes/git_subtree_spec.rb:33:in `block (3 levels) in <top (required)>'

  3) git::subtree when git version >= 1.7.11 should contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core] with creates => "/usr/share/doc/git/contrib/subtree/git-subtree" and cwd => "/usr/share/doc/git/contrib/subtree"
     Failure/Error: it { should create_exec('/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core').with({
       expected that the catalogue would contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core]
     # ./spec/classes/git_subtree_spec.rb:53:in `block (3 levels) in <top (required)>'

  4) git::subtree when git version >= 1.7.11 should contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core install] with creates => "/usr/lib/git-core/git-subtree" and cwd => "/usr/share/doc/git/contrib/subtree"
     Failure/Error: it { should create_exec('/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core install').with({
       expected that the catalogue would contain Exec[/usr/bin/make prefix=/usr libexecdir=/usr/lib/git-core install] with creates set to "/usr/lib/git-core/git-subtree" but it is set to nil, and parameter cwd set to "/usr/share/doc/git/contrib/subtree" but it is set to "/contrib/subtree"
       Diff:
       @@ -1,4 +1,2 @@
       -/usr/lib/git-core/git-subtree
       -
       -/usr/share/doc/git/contrib/subtree
       +/contrib/subtree
     # ./spec/classes/git_subtree_spec.rb:58:in `block (3 levels) in <top (required)>'
```

This makes things green :+1: