Bumped conda-build

This MR solves the remaining issue for using conda-build 3.18 (and ripgrep in particular), in our builds.

Closes #32 (closed)

bob/devtools/bootstrap.py

@@ -448,7 +448,7 @@ if __name__ == "__main__":
                 ))
 
     conda_version = "4"
-    conda_build_version = "3.17"
+    conda_build_version = "3"
     conda_verify_version = "3"
 
     conda_verbosity = []