Travis: fix scandcode failure

This is workaround for https://github.com/ARMmbed/mbed-os/issues/13322. We can remove
this line once they fix the dependency.

.travis.yml

@@ -58,6 +58,8 @@ matrix:
       language: python
       python: 3.6.8 # scancode-toolkit v3.1.1 requires v3.6.8
       install:
+        # workaround for https://github.com/ARMmbed/mbed-os/issues/13322
+        - pip install pdfminer.six==20200517
         - pip install scancode-toolkit==3.1.1
       before_script:
         - mkdir -p SCANCODE