Don't install ReportLab under PyPy on TravsiCI

One of our unit tests crashes in the cpyext layer of PyPy:

test_ColorSpiral ... Fatal error in cpyext, CPython compatibility layer, calling PySequence_GetItem
Either report a bug or consider not using this particular extension
<OpErrFmt object at 0x7f23639a48f0>
RPython traceback:
  File pypy_module_cpyext_api_1.c, line 32086, in PySequence_GetItem
  File pypy_module_cpyext_pyobject.c, line 1060, in BaseCpyTypedescr_realize
  File pypy_objspace_std_objspace.c, line 4334, in allocate_instance__W_ObjectObject
  File pypy_objspace_std_typeobject.c, line 3750, in W_TypeObject_check_user_subclass
/home/travis/build.sh: line 317:  9032 Segmentation fault      python run_tests.py --offline