Install PyCrypto via pip for Travis CI build

PyCrypto is currently not available when running the vpicc unittest so
the tests fails. In order to run the tests sucessfully we install
PyCrypto via pip. This only works only works on linux and only when not
using mingw32-gcc.

.travis.yml

@@ -29,6 +29,7 @@ install:
  if [ -z "$HOST" ]; then
  sudo ln -s /lib/x86_64-linux-gnu/libgcc_s.so.1 /lib/x86_64-linux-gnu/libgcc_s.so;
  sudo apt-get -qq install libpcsclite-dev libusb-dev xutils-dev gengetopt help2man swig python-dev;
+ pip install --user pycrypto;
  else
  sudo apt-get -qq install libpcsclite-dev libusb-dev xutils-dev gengetopt help2man binutils-mingw-w64 gcc-mingw-w64;
  fi