Bump pyyaml from 3.12 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -56,7 +56,7 @@ psycopg2-binary==2.7.4
 pybloom-live==2.3.1       # via tableschema-sql
 python-dateutil==2.6.1    # via botocore, datapackage-pipelines, tableschema
 pytz==2017.3              # via celery
-pyyaml==3.12              # via datapackage-pipelines
+pyyaml==5.1               # via datapackage-pipelines
 readchar==2.0.1           # via inquirer
 redis==3.3.11             # via datapackage-pipelines, os-api-cache
 regex==2017.9.23          # via awesome-slugify