Downgrade and filter PyYAML 4.1

This reverts commit 96defc5dc2.

With PyYAML 4.1 we don't have C extensions on Travis CI:
https://github.com/yaml/pyyaml/issues/179
https://github.com/yaml/pyyaml/issues/182

Unfortunately, cython isn't in the APT whitelist either:
https://github.com/travis-ci/apt-source-whitelist/issues/37

misc/requirements/requirements-qutebrowser.txt-raw

@@ -1,7 +1,9 @@
 Jinja2
 Pygments
 pyPEG2
-PyYAML
+PyYAML!=4.1
 colorama
 cssutils
 attrs
+
+#@ filter: PyYAML != 4.1

misc/requirements/requirements-tests-git.txt

@@ -35,4 +35,8 @@ git+https://github.com/pallets/markupsafe.git
 hg+http://bitbucket.org/birkenfeld/pygments-main
 hg+https://bitbucket.org/fdik/pypeg
 git+https://github.com/python-attrs/attrs.git
-git+https://github.com/yaml/pyyaml.git
+
+# Fails to build:
+# gcc: error: ext/_yaml.c: No such file or directory
+# hg+https://bitbucket.org/xi/pyyaml
+PyYAML==3.12

requirements.txt

@@ -7,4 +7,4 @@ Jinja2==2.10
 MarkupSafe==1.0
 Pygments==2.2.0
 pyPEG2==2.15.2
-PyYAML==4.1
+PyYAML==3.12  # rq.filter: != 4.1