Fixes #212: ensure pact path is consistent

[solarmosaic-kflorence]

See #212 -- it seems that when given a new File(relativePath), file.createNewFile() will create a file in a different place for SBT sub-projects and root-projects. Using new File(absolutePath) instead resolves this issue.

I'm not sure the best way to add test coverage for this, but it can be reproduced by:

• creating an SBT project that has a sub-project and a root project both with .enablePlugins(ScalaPactPlugin).
• creating a consumer pact in both projects and and running tests against them (sbt pactTest, assuming you .aggregate the sub-project in the root project)
• you should see pact files get generated in the root target/pacts folder, as well as in subProject/target/pacts prior to this fix, and after this fix they should both be in the root target/pacts folder.