Conda tab raising errors following conda update

[GPhilo]

Due to conda not being in PATH anymore, the Conda tab is raising errors in newly-created environments (See this issue for nb_conda_kernels for more details on what the change in conda was exactly and how it broke the package).

The fix, however, is simply to replace references to conda with the value of the environment variable CONDA_EXE.

[GPhilo]

Ah, I see now that there is already a PR for a fix: #75