Closes #227: adds PyMySQL in install/requirements.txt

.travis.yml

   - sh -c "if [ '$DB' = 'mysql' ]; then mysql -e 'CREATE DATABASE tracim_test;'; fi"
   - sh -c "if [ '$DB' = 'mysql' ]; then cd ${TRAVIS_BUILD_DIR}/tracim && sed -i \"s/\(sqlalchemy.url *= *\).*/\mysql+pymysql:\/\/root@localhost\/tracim_test/\" test.ini; fi"
   - sh -c "if [ '$DB' = 'mysql' ]; then cd ${TRAVIS_BUILD_DIR}/tracim && sed -i \"s/<replace_database_uri_here>/mysql+pymysql:\/\/root@localhost\/tracim_test/\" development.ini; fi"
-  - sh -c "if [ '$DB' = 'mysql' ]; then pip install PyMySQL; fi"
 
   - sh -c "if [ '$DB' = 'sqlite' ]; then cd ${TRAVIS_BUILD_DIR}/tracim && sed -i \"s/\(sqlalchemy.url *= *\).*/\sqlite:\/\/\/tracim_test.sqlite/\" test.ini; fi"
   - sh -c "if [ '$DB' = 'sqlite' ]; then cd ${TRAVIS_BUILD_DIR}/tracim && sed -i \"s/<replace_database_uri_here>/sqlite:\/\/\/tracim.sqlite/\" development.ini; fi"

install/requirements.txt

 typing==3.5.3.0
 rq==0.7.1
 click==6.7
+PyMySQL==0.7.11