Fix for python2.7 environment

Python envs used till now python3 for testing, now version of python
depends on the environment - for py2* python 2.7 will be used, and for
py3* appropriate python3 version will be used.

Also, added mock module for python2 envs.

tests/test_client.py

@@ -1,5 +1,8 @@
 from unittest import TestCase
-from unittest.mock import MagicMock
+try:
+    from unittest.mock import MagicMock
+except ImportError:
+    from mock import MagicMock
 
 from slack_backup import client
 from slack_backup import objects as o

tox.ini

@@ -4,13 +4,19 @@ envlist = py27,py34,{py27,py34}-flake8
 usedevelop = True
 
 [testenv]
-basepython = python3.4
 usedevelop=True
 setenv = COVERAGE_FILE = .coverage
 commands = py.test --cov=slack_backup --cov-report=term-missing
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
+[testenv:py27]
+usedevelop={[testenv]usedevelop}
+setenv = {[testenv]setenv}
+commands = {[testenv]commands}
+deps = {[testenv]deps}
+        mock
+
 [testenv:py27-flake8]
 basepython = python2.7
 deps = flake8