Check url and version options are not both in options

This makes install() idempotent as it should be.
And by default, the user expects recipes to do their best
to undo any partial change on error.

Traceback (most recent call last):
  ...
    recipe.install()
  File "rubygemsrecipe/rubygems.py", line 347, in install
    gem_executable):
  File "rubygemsrecipe/rubygems.py", line 300, in get_dependency_list
    if self.version[0] < '3':
TypeError: 'NoneType' object has no attribute '__getitem__'

The test that was added in 60b10788 is adapted due to the call to subprocess.checkout .

See merge request !7

Add test reproducing a crash in deployment mode when rubygems is already installed

Handle gem version exclusion from gemspec ( != symbol )

/reviewed-by @kirr
/reviewed-on !6

Search pattern for a gem name for rubygems > 3.0 is in format ^gem_name$ instead
of /^gem_name$/ for rubygems < 3.0

/reviewed-by @kirr
/reviewed-on !6

Fix gems constraints detection failures when similar version constraint ( ~> )
is used as second constraint by extending the regex.

/reviewed-by @kirr
/reviewed-on !6

Store return value of command 'gem dependency' in files rather than using a
dictionnary to use the same output as when the check_output method is not mocked

/reviewed-by @kirr
/reviewed-on !6

Using decode method on subprocess.checkoutput() throws an encoding error
in Python2 when returned value contains non ascii caracters

/reviewed-by @kirr
/reviewed-on !6

If deployment mode option is not provided, default
value is `not allow-picked-version`.

- adds deployment mode in recipe options
- checks errors message in tests

tests_require is only supported by setuptools' test entry point which is
deprecated, extras_require is supported by most tools.

Adds option --no-document to install command to reproduce behavior
before commit c58aba06 with Rubygems
version >= 2.0.0 .

Update tests to comply with commits:
- 904ef9ec (--no-rdoc and --no-ri
options deletion)
- c58aba06 (adds ruby executable path to commands)

hexagonit.recipe.download having not been ported to Python3, changing
for slapos.recipe.downloadunpacked is a Python3 portage for this recipe.

RubyGems 2.0.0preview2 removed --no-rdoc and --no-ri options
for the install command.

(https://github.com/rubygems/rubygems/blob/master/History.txt)

Ruby executable is normally available in gem_executable shebang
but shebang's length is limited to 128 bytes (Linux < 5.1)
so it can not be used for very long executable path.

https://bitbucket.org/sirex/rubygemsrecipe/issue/6/cannot-install-rubygems-020

While fixing failing tests, did some code refactoring.

make python3 compatible (fixes #3)

Add option to sepcify rubygems url now that download location has changed.