temporarily pin coverage to version < 4.2

as otherwise coverage combine removes content of .coverage
see https://bitbucket.org/ned/coveragepy/issues/511

tox.ini

@@ -24,7 +24,7 @@ setenv =
 passenv = *
 usedevelop = false
 deps =
-    coverage
+    coverage<4.2
     mock
     nose
     notebook40: notebook>=4.0,<4.1
@@ -76,7 +76,9 @@ commands =
 
 [testenv:coveralls]
 skip_install = true
-deps = coveralls
+deps =
+    coverage<4.2
+    coveralls
 commands =
     coverage combine
     coverage report
@@ -85,6 +87,7 @@ commands =
 [testenv:codecov]
 skip_install = true
 deps =
+    coverage<4.2
     codecov
 commands =
     coverage combine
@@ -97,7 +100,7 @@ skip_install = true
 whitelist_externals = bash
 deps =
     appveyor-artifacts
-    coverage
+    coverage<4.2
     coveralls
 commands =
     appveyor-artifacts --owner-name=jcb91 --repo-name=ipython-notebook-extensions-ynb9f --mangle-coverage download
@@ -110,7 +113,7 @@ commands =
 
 [testenv:report]
 skip_install = true
-deps = coverage
+deps = coverage<4.2
 commands =
     coverage combine
     coverage report
@@ -118,7 +121,7 @@ commands =
 
 [testenv:clean]
 skip_install = true
-deps = coverage
+deps = coverage<4.2
 commands = coverage erase
 
 [testenv:bump]