bump traivs and docker files to use rust 1.42.0

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 
 rust:
-  - 1.41.1
+  - 1.42.0
 
 matrix:
   include:

devops/dockerfiles/rust-builder/Dockerfile

@@ -1,4 +1,4 @@
-FROM liuchong/rustup:1.41.1 AS builder
+FROM liuchong/rustup:1.42.0 AS builder
 LABEL description="Rust and WASM build environment for joystream and substrate"
 
 WORKDIR /setup