Use multi dimension from Tox 1.8.0

 * update tox from 1.7.2 to 1.8.0
 * small style change - add spaces around = in tox.ini
 * rewrite tox.ini with support for multi dimension environment
   specification - less repetition

Closes #83
Merge pull request #93 from ravwojdyla/upgrade_tox

Use multi dimension from Tox 1.8.0