Release v0.6.4 (#442)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Release steps for v0.6.4.

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [N/A] I have added a one-line description of my change to the
changelog in
      `HISTORY.md`
- [x] This PR is ready to go

ribs/__init__.py

@@ -2,7 +2,7 @@
 
 __author__ = "ICAROS Lab pyribs Team"
 __email__ = "[email protected]"
-__version__ = "0.6.3"
+__version__ = "0.6.4"
 
 from ribs import archives, emitters, schedulers
 

setup.cfg

@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 0.6.3
+current_version = 0.6.4
 commit = True
 tag = False
 

setup.py

@@ -102,6 +102,6 @@
     python_requires=">=3.8.0",
     test_suite="tests",
     url="https://github.com/icaros-usc/pyribs",
-    version="0.6.3",
+    version="0.6.4",
     zip_safe=False,
 )