[AIRFLOW-1430] Solve GPL dependency

One of the dependencies was pulling in
a GPL library by default. With the new
release of python-nvd3 this is now solved.

Closes apache#3160 from bolkedebruin/legal

docs/installation.rst

@@ -15,6 +15,14 @@ You can also install Airflow with support for extra features like ``s3`` or ``po
 .. code-block:: bash
 
     pip install "apache-airflow[s3, postgres]"
+    
+.. note:: GPL dependency
+
+    One of the dependencies of Apache Airflow by default pulls in a GPL library ('unidecode').
+    In case this is a concern you can force a non GPL library by issuing
+    ``export SLUGIFY_USES_TEXT_UNIDECODE=yes`` and then proceed with the normal installation.
+    Please note that this needs to be specified at every upgrade. Also note that if `unidecode`
+    is already present on the system the dependency will still be used.
 
 Extra Packages
 ''''''''''''''
@@ -107,4 +115,4 @@ After configuration, you'll need to initialize the database before you can run t
 
 .. code-block:: bash
 
-    airflow initdb
+    airflow initdb

setup.py

@@ -240,7 +240,7 @@ def do_setup():
             'pygments>=2.0.1, <3.0',
             'python-daemon>=2.1.1, <2.2',
             'python-dateutil>=2.3, <3',
-            'python-nvd3==0.14.2',
+            'python-nvd3==0.15.0',
             'requests>=2.5.1, <3',
             'setproctitle>=1.1.8, <2',
             'sqlalchemy>=1.1.15, <1.2.0',