Update vuepress to fix hot reloading

- Version 1.5 had an issue with hot reloading that was resolved in
version 1.5.1
- Update vue and vue-template-compiler to vue@2.6.12 to resolve
vuepress dependencies

Testing:
- Ran docs locally (npm run docs:serve) and verified all pages
- Ran app locally (npm run serve) and spot tested several pages

Signed-off-by: Derick Montague <derick.montague@ibm.com>
Change-Id: Iec3494e00756040d9ce2f9ae6d1475f37dba0032

Author: derick-montague