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 nexedi/rubygemsrecipe!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 nexedi/rubygemsrecipe!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.