Don't require too old mbed tools

Don't require too old of a version of mbed-greentea, mbed-host-tests, or
mbed-ls. We conservatively limit the versions to less than 2.0 for now,
in case we'ld like to break backwards compatibility in the future.

Author: Patater

requirements.txt

@@ -12,9 +12,9 @@ idna>=2,<2.8
 pyserial>=3,<=3.4
 Jinja2>=2.10.1,<2.11
 intelhex>=1.3,<=2.2.1
-mbed-ls>=1.5.1,<1.8
-mbed-host-tests>=1.4.4,<1.6
-mbed-greentea>=0.2.24,<1.8
+mbed-ls>=1.5.1,<2.0
+mbed-host-tests>=1.4.4,<2.0
+mbed-greentea>=0.2.24,<2.0
 beautifulsoup4>=4,<=4.6.3
 pyelftools>=0.24,<=0.25
 manifest-tool==1.5.2