fix dependenci and improve doc (#1535)

apps/android_rpc/README.md

docs/api/python/autotvm.rst

docs/install/from_source.rst

docs/vta/dev/hardware.rst

python/tvm/autotvm/measure/measure_methods.py

python/tvm/autotvm/record.py

python/tvm/exec/rpc_tracker.py

python/tvm/rpc/__init__.py

tutorials/autotvm/tune_conv2d_cuda.py

tutorials/autotvm/tune_nnvm_arm.py

tutorials/cross_compilation_and_rpc.py