require treetime version >=0.11.2

This is required for VCF related tests added in <#1355>

Note that different parts of our CI can use different TreeTime versions
according to their installation method. For instance, "pytest-cram"
installs Augur & TreeTime via pip, thus getting 0.11.2 at the time of
writing. "pathogen-repo-ci" first installs augur via conda, and thus
gets 0.11.1.

setup.py

@@ -62,7 +62,7 @@
         "numpy ==1.*",
         "packaging >=19.2",
         "pandas >=1.0.0, ==1.*",
-        "phylo-treetime >=0.10.0, <0.12",
+        "phylo-treetime >=0.11.2, <0.12",
         "pyfastx >=1.0.0, <3.0",
         "scipy ==1.*",
         "xopen[zstd] >=1.7.0, ==1.*"