Update installation instructions

[TobiasDuswald]

I tried to build the package because the pip installation fails, see issue eth-cscs#111. When building it with

make package
pip3 install wheel
pip3 install build/dist/abcpy-0.6.3-py3-none-any.whl

I received errors saying of type

>python3 setup.py -v bdist_wheel -d build/dist
command not found `bidet_wheel

Switching the order of the statements fixed it.

pip3 install wheel
make package
pip3 install build/dist/abcpy-0.6.3-py3-none-any.whl