Add official support for elasticsearch 1.6

Author: bmermet

docs/index.rst

@@ -448,7 +448,7 @@ We officially support Python 2.7, 3.4 and above.
 +-----------------+--------------------+
 | django          | >= 1.8             |
 +-----------------+--------------------+
-| elasticsearch   | >= 2.3             |
+| elasticsearch   | >= 1.6             |
 +-----------------+--------------------+
 | falcon          | >= 1.0             |
 +-----------------+--------------------+

tox.ini

@@ -25,7 +25,7 @@ envlist =
     {py27,py34,py35,py36}-bottle-autopatch{12}-webtest
     {py27,py34,py35,py36}-cassandra{35,36,37,38}
     {py27,py34,py35,py36}-celery{31,40}-redis{210}
-    {py27,py34,py35,py36}-elasticsearch{23,24,51,52}
+    {py27,py34,py35,py36}-elasticsearch{16,17,18,23,24,51,52,53,54}
     {py27,py34,py35,py36}-falcon{10,11,12}
     {py27,py34,py35,py36}-falcon-autopatch{10,11,12}
     {py27,py34,py35,py36}-django{18,19,110,111}-djangopylibmc06-djangoredis45-pylibmc-redis{210}-memcached
@@ -125,10 +125,15 @@ deps =
     celery31: celery>=3.1,<3.2
     celery40: celery>=4.0,<4.1
     ddtracerun: redis
+    elasticsearch16: elasticsearch>=1.6,<1.7
+    elasticsearch17: elasticsearch>=1.7,<1.8
+    elasticsearch18: elasticsearch>=1.8,<1.9
     elasticsearch23: elasticsearch>=2.3,<2.4
     elasticsearch24: elasticsearch>=2.4,<2.5
     elasticsearch51: elasticsearch>=5.1,<5.2
     elasticsearch52: elasticsearch>=5.2,<5.3
+    elasticsearch53: elasticsearch>=5.3,<5.4
+    elasticsearch54: elasticsearch>=5.4,<5.5
     falcon10: falcon>=1.0,<1.1
     falcon11: falcon>=1.1,<1.2
     falcon12: falcon>=1.2,<1.3
@@ -218,7 +223,7 @@ commands =
     bottle-autopatch{12}: ddtrace-run nosetests {posargs} tests/contrib/bottle/test_autopatch.py
     cassandra{35,36,37,38}: nosetests {posargs} tests/contrib/cassandra
     celery{31,40}: nosetests {posargs} tests/contrib/celery
-    elasticsearch{23,24,51,52}: nosetests {posargs} tests/contrib/elasticsearch
+    elasticsearch{16,17,18,23,24,25,51,52,53,54}: nosetests {posargs} tests/contrib/elasticsearch
     django{18,19,110,111}: python tests/contrib/django/runtests.py {posargs}
     django-autopatch{18,19,110,111}: ddtrace-run python tests/contrib/django/runtests.py {posargs}
     flaskcache{012,013}: nosetests {posargs} tests/contrib/flask_cache