Move coverage to test dependency and install it properly

- Coverage should only be required if running tests

- When running test_coverage command all the install dependencies and
  test dependencies should be installed.

plinth/tests/coverage/test_coverage.py

@@ -68,8 +68,14 @@ class TestCoverageCommand(setuptools.Command):
         """
         Main command implementation.
         """
+        if self.distribution.install_requires:
+            self.distribution.fetch_build_eggs(
+                self.distribution.install_requires)
 
         # erase any existing HTML report files
+        if self.distribution.tests_require:
+            self.distribution.fetch_build_eggs(self.distribution.tests_require)
+
         try:
             shutil.rmtree(COVERAGE_REPORT_DIR, True)
         except:

setup.py

@@ -114,9 +114,9 @@ setuptools.setup(
     install_requires=[
         'cherrypy >= 3.0',
         'django >= 1.7.0',
-        'django-bootstrap-form',
-        'coverage >= 3.7'
+        'django-bootstrap-form'
     ],
+    tests_require=['coverage >= 3.7'],
     include_package_data=True,
     package_data={'plinth': ['templates/*',
                              'modules/*/templates/*']},