Update version to 1.3.7

We have agreed to keep all PEcAn module versions in sync, e.g. all of the outputs from the following command should currently be at 1.3.7:

This required:

```sh
git checkout -b update_version
sed -i 's/1.3.3/1.3.7/g' scripts/updateDescription.sh
./scripts/updateDescription.sh
git push origin update_version
```

... not sure if there is a way to automate this. Its not difficult, but should be part of the version release.

And ... not sure if we want to add this to the 1.3.7 tag.
In general if we don't need to order the files, or ignore files, it is best to not add a Collate section, see http://cran.r-project.org/doc/manuals/R-exts.html#The-DESCRIPTION-file.