Add XBlock 1.2 to test matrix

XBlock 1.2.0 was released on 2018-03-29, XBlock 1.2.1 on 2018-06-26.

Since it doesn't make a whole lot of sense to test on outdated
point releases, add just XBlock 1.2.1 to the test matrix. Also,
rename the other environments xblock-10, 11, and 12 to simplify
future updates.

Reference:
https://pypi.org/project/XBlock/#history

tox.ini

@@ -2,12 +2,12 @@
 #
 # Until that is fixed, run tests on Python 3.5 but ignore errors.
 [tox]
-envlist = py{27,35}-xblock{100,111},flake8
+envlist = py{27,35}-xblock{10,11,12},flake8
 
 [travis]
 python =
-  2.7: py27-xblock{100,111},flake8
-  3.5: py35-xblock{100,111},flake8
+  2.7: py27-xblock{10,11,12},flake8
+  3.5: py35-xblock{10,11,12},flake8
 
 [flake8]
 ignore = E124
@@ -16,8 +16,9 @@ exclude = .svn,CVS,.bzr,.hg,.git,__pycache__,.tox,build,dist,src
 [testenv]
 deps =
     -rrequirements/test.txt
-    xblock100: XBlock==1.0.0
-    xblock111: XBlock==1.1.1
+    xblock10: XBlock==1.0.0
+    xblock11: XBlock==1.1.1
+    xblock12: XBlock==1.2.1
 commands =
     py27: python run_tests.py []
     py35: - python run_tests.py []