If an f-string was ended with a newline character (more precisely, if
the last element of a JoinedStr node was just a newline character), its
round-trip source code representation would have an extra newline on
a wrong place, resulting in invalid syntax.

The bug affects only python>=3.6 (f-strings weren't supported before)

This commit fixes the aforementioned bug.

This commit also replaces pypy3.3-5.2-alpha1 tox/travis with pypy3.5.

The former PyPy version has a bug causing the tests to fail:

https://bitbucket.org/pypy/pypy/issues/2316/osget_terminal_size-is-missing-on-pypy3-hg

Fixes #89.