Updated the package version in CMakeLists.txt

Summary: This should be `3.4.0` in the 3.4 branch.
Closes https://github.com/facebook/hhvm/pull/4181

Reviewed By: @JoelMarcey

Differential Revision: D1669153

Signature: t1:1669153:1415514126:b9abcf702f328ac51c6ead9966b5d43238d788fe