Update nbval requirement from >=0.9.0 to >=0.11.0

Updates the requirements on [nbval](https://github.com/computationalmodelling/nbval) to permit the latest version.
- [Commits](computationalmodelling/nbval@0.9...0.11.0)

---
updated-dependencies:
- dependency-name: nbval
  dependency-version: 0.11.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ scipy>=0.16
 numba
 
 pytest>=3.6
-nbval>=0.9.0
+nbval>=0.11.0
 pytest-cov>=2.6.0
 
 flake8>=3.5.0