travis-ci: switch to trusty; cache pip packages

This applies several changes to the non-docker travis-ci builds:

- Make all builds use trusty rather than precise.  pybind can't really
  build in precise anyway (we install essentially the entire toolchain
  backported from trusty on every build), and so this saves needing to
  install all the backported packages during the build setup.
- Updated the 3.5 build to 3.6 (via deadsnakes, which didn't backport
  3.6 to ubuntu releases earlier than trusty).
- As a result of the switch to trusty, the BAREBONES build now picks up
  the (default installed) python 3.5 installation.
- Invoke pip everywhere via $PYTHON -m pip rather than the pip
  executable, which saves us having to figure out what the pip
  executable is, and ensures that we are using the correct pip.
- Install packages with `pip --user` rather than in a virtualenv.
- Add the local user python package archive to the travis-ci cache
  (rather than the pip cache).  This saves needing to install packages
  during installation (unless there are updates, in which case the
  package and the cache are updated).
- Install numpy and scipy on the pypy build.  This has to build from
  source (and so blas and fortran need to be installed on the build),
  but given the above caching, the build will only be slow for the first
  build after a new numpy/scipy release.  This testing is valuable:
  numpy has various behaviour differences under pypy.
- Added set -e/+e around the before_install/install blocks so that a
  failure here (e.g. a pip install failure or dependency download
  failure) triggers a build failure.
- Update eigen version to latest (3.3.3), mainly to be consistent with
  the appveyor build.
- The travis trusty environment has an upgraded cmake, so this
  downgrades cmake (to the stock trusty version) on the first couple
  jobs so that we're still including some cmake 2.8.12 testing.