Bump problem-builder to v2.6.5.

[adampalay]

It includes support for course keys longer than 50 characters.

See: open-craft/problem-builder#131

@sanfordstudent FYI
@mtyaka , how do we verify this on stage?

[mtyaka]

@adampalay Steps to verify:

1. Create a new course. Give it a long course name/org/run, so that the course key is longer than 50 characters.
2. Add a Problem Builder block to the course, then add a Long Answer child component to the Problem builder block.
3. Go to the unit in LMS. In old version of problem-builder the LMS throws an error; new version works correctly.

[adampalay]

jenkins run bokchoy

[sanfordstudent]

jenkins run all