Bug 1595218 - Update bootstrap to upgrade to stable rust. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D53900

--HG--
extra : moz-landing-system : lando

python/mozboot/mozboot/base.py

@@ -158,7 +158,7 @@ MODERN_MERCURIAL_VERSION = LooseVersion('4.8')
 MODERN_PYTHON_VERSION = LooseVersion('2.7.3')
 
 # Upgrade rust older than this.
-MODERN_RUST_VERSION = LooseVersion('1.37.0')
+MODERN_RUST_VERSION = LooseVersion('1.39.0')
 
 # Upgrade nasm older than this.
 MODERN_NASM_VERSION = LooseVersion('2.14')