Try with
!git clone https://bitbucket.org/unilucompmech/fenics-shells.git && cd fenics-shells && find ./ -type f -exec sed -i 's|import ufl|import ufl_legacy|g' {} \; && find ./ -type f -exec sed -i 's|from ufl|from ufl_legacy|g' {} \; && pip install --no-dependencies .
The two find instructions are due to Announcement: ufl_legacy and legacy dolfin, while --no-dependencies is to avoid downgrading fenics components from 2019.2.0.dev0 (packaged on Colab) to 2019.1.0. @jackhale may be able to better tell if the latter is supposed to work e.g. in all demos, but at least import fenics_shells does work for me.
@jackhale will also surely tell you that in future you may want to consider FEniCSx-Shells instead, see FEniCSx-Shells — FEniCSx-Shells 0.5.0.dev0 documentation 