Bump pyyaml from 3.11 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

req-rtd.txt

@@ -1,5 +1,5 @@
 picklable-itertools==0.1.1
 progressbar2==3.6.2
-pyyaml==3.11
+pyyaml==5.1
 requests==2.20.0
 six==1.10.0

req-travis-conda.txt

@@ -5,7 +5,7 @@ nose==1.3.7
 numpy==1.10.4
 pillow==5.2.0
 pytables==3.4.4
-pyyaml==3.13
+pyyaml==5.1
 pyzmq==15.2.0
 scipy==0.17.0
 six==1.10.0

requirements.txt

@@ -4,7 +4,7 @@ tables==3.4.4
 picklable-itertools==0.1.1
 pillow==5.2.0
 progressbar2==3.6.2
-pyyaml==3.13
+pyyaml==5.1
 pyzmq==15.2.0
 requests==2.20.0
 scipy==0.17.0