Enable PyPy builds on Travis

Travis now supports PyPy 2.0.
Revert "Enable PyPy builds on Travis"

This reverts commit 15eb1e3ae04dada1267dc1e30a6566a2c78328ed.
session: Use convert() helper to shorten code
error: Allow  to be any iterable
loadable: Add 'timeout' argument
loadable: Return self from load() to support chaining