Back to being custom, since the 'Rust' image has a really old Python version

.travis.yml

@@ -1,16 +1,14 @@
-language: rust
-
-rust:
-  - stable
-  - beta
+language: python
 
 env:
-  - LUA="lua=5.1"
+  - RUST="stable" LUA="lua=5.1"
+  - RUST="beta" LUA="lua=5.1"
 
 before_install:
   - pip install hererocks
   - hererocks lua_install -r^ --$LUA
-  - export PATH=$PATH:$PWD/lua_install/bin
+  - curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain $RUST
+  - export PATH=$PATH:$PWD/lua_install/bin:$HOME/.cargo/bin
 
 install:
   - luarocks install luafilesystem