Travis: Install frappe of targetted branch

For example:
If pull request is for wip-4.1 branch of erpnext, install frappe with branch as wip-4.1
diff --git a/.travis.yml b/.travis.yml
index 59acb5c..dba0dab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,9 +14,13 @@
   - sudo apt-get update
   - sudo apt-get purge -y mysql-common
   - sudo apt-get install mariadb-server mariadb-common libmariadbclient-dev
-  - CFLAGS=-O0 pip install git+https://github.com/frappe/frappe.git@develop &&
+  - CFLAGS=-O0 pip install git+https://github.com/frappe/frappe.git@$TRAVIS_BRANCH &&
   - pip install --editable .
 
+before_script:
+  - mysql -e 'create database test_frappe'
+  - echo "USE mysql;\nCREATE USER 'test_frappe'@'localhost' IDENTIFIED BY 'test_frappe';\nFLUSH PRIVILEGES;\n" | mysql -u root
+
 script:
   - cd ./test_sites/
   - frappe --use test_site
@@ -25,7 +29,3 @@
   - frappe -b
   - frappe --serve_test &
   - frappe --verbose --run_tests --app erpnext
-
-before_script:
-  - mysql -e 'create database test_frappe'
-  - echo "USE mysql;\nCREATE USER 'test_frappe'@'localhost' IDENTIFIED BY 'test_frappe';\nFLUSH PRIVILEGES;\n" | mysql -u root