Remove unneeded package from requirements

The presence of the package pkg-resources is the result of a bug from
debian based OS:
pypa/pip#4022
It can cause installation failure on other OS (Archlinux for instance).

requirements/requirement.txt

@@ -41,7 +41,6 @@ odfpy==1.3.6
 olefile==0.45.1
 # moved to repanier.packages : openpyxl==1.8.6, with bug fix on worker.worksheet.py lines 103-106
 openpyxl==2.5.5
-pkg-resources==0.0.0
 psycopg2==2.7.5
 pyaml==17.8.0
 python-dateutil==2.7.3
@@ -54,4 +53,4 @@ tablib==0.11.5
 uWSGI==2.0.17.1
 unicodecsv==0.14.1
 xlrd==1.0.0
-xlwt==1.2.0
+xlwt==1.2.0