Don't let Rust pick 1.6.x in place of 1.5.x.

NCS releases are not semantically versioned and a 1.6.x might
include breaking changes, so we don't want to pick it up
automatically.

Author: Jonathan Pallant (42 Technology)

Cargo.toml

@@ -15,4 +15,4 @@ cortex-m = "0.6"
 heapless = "0.7"
 linked_list_allocator = { version="0.9.0", default-features=false, features=["use_spin"] }
 log = "0.4"
-nrfxlib-sys = "1.5.1"
+nrfxlib-sys = "=1.5.1"