…e deprecated `pip install --download` support.

Among other changes,
- Introduced a set of global variables named `pip_version`, `pip_download_command` and `pip_no_binary_command`. The variables `pip_download_command` and `pip_no_binary_command` are version dependent.
- Now we are importing the `main()` method from `pip` (for pip<10.0.0) or from `pip._internal` (for pip>=10.0.0) as `pip_main`.
- Introduced a new flag `has_pip` (pretty much like `has_wheel`) as a global flag to denote the presence of pip.
- The help messages printed by the pip2pi parser now dynamically prints options for pip download and no-binary features based on installed pip version.

Get Travis CI tests to run properly.

…ution of each pip command.

When populated by one `pip_main` call, the variable remains in the environment properties for all subsequent calls, wreaking havoc since the directory is subsequently deleted.
Should be removed once fixed in `pip`, but will remain until then.

Also removed testing for py33 as pip 18.1 demands 3.4.