Replace custom build of Z3 with z3-turnkey

[romac]

The purpose of z3-turnkey is to provide a JAR artifact that:

1. ships its own native libraries,
2. can use them without administrative privileges, and
3. can be obtained using Maven.

If this works, this could allow us to distribute ScalaZ3 via Maven or Bintray, without requiring that users manually build it themselves or download one of our pre-built artifacts.

[Dessix]

This may be easier now that #88 no longer requires patching of the Z3 sources.