merge branch 'bump_versions'

Bump required Python version and library dependencies to the
oldest supported ones. With the release of Debian Stretch
the oldest supported platform is Ubuntu 16.04, which has:

+ Python 3.5
+ numpy 0.11.0
+ scipy 0.17.0
+ matplotlib 1.5.1

We are making these changes now in the expectation that we
will not make a new release before 6 months have elapsed, in
line with our policy to support old platforms for 6 months
after they are supplanted.

Closes #31

See merge request !146