remove pypy from the travis builds (failure here https://travis-ci.org/giampaolo/pysendfile/jobs/26119015)

test_offset_none (__main__.TestSendfile) ... Fatal error in cpyext,
CPython compatibility layer, calling PyEval_SaveThread
Either report a bug or consider not using this particular extension
<SystemError object at 0x1c7bf20>
RPython traceback:
  File "pypy_interpreter_pyopcode.c", line 6476, in
dispatch_bytecode__AccessDirect_None
  File "implement.c", line 23562, in __mm_nonzero_0_fail