ruby-dev required on debian-based distros (ubuntu, mint) to install gem with native ext

[robertocabellon]

On Linux Mint 18.1 (based on Ubuntu 16.04) have returned the following...

$ sudo gem install gitdocs
Building native extensions.  This could take a while...
ERROR:  Error installing gitdocs:
	ERROR: Failed to build gem native extension.

    current directory: /var/lib/gems/2.3.0/gems/hitimes-1.2.4/ext/hitimes/c
/usr/bin/ruby2.3 -r ./siteconf20170519-8357-1yha5b8.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.3.0/gems/hitimes-1.2.4 for inspection.
Results logged to /var/lib/gems/2.3.0/extensions/x86_64-linux/2.3.0/hitimes-1.2.4/gem_make.out

I've fixed up by apt-installing the ruby2.3-dev package.

You may mention this requirement for debian-based distros on the README.

thanks to @copiousfreetime ! https://github.com/copiousfreetime/hitimes/issues/45#issuecomment-117157913

[robertocabellon]

among other dependencies, that are well explicited on the returned messages during installation... like: libsqlite3-dev and cmake