Fix issue with shebang length being too long in pip executable (#872)

Author: tnguyen14

tests/bootstrap.sh

@@ -8,9 +8,9 @@ if [ "${TRAVIS:-false}" = "false" ]; then
 fi
 
 
-pip install selenium===3.14.1 \
-            docker===3.5.0 \
-            | grep -v 'Requirement already satisfied'
+python -m pip install selenium===3.14.1 \
+                      docker===3.5.0 \
+                      | grep -v 'Requirement already satisfied'
 
 python test.py $1 $2
 ret_code=$?