Replaced std with core in the TryInto import (#181)

* Replaced std with core

* Changed rust nightly to not allowed to fail in travis CI

.travis.yml

@@ -6,9 +6,6 @@ language:
 rust:
   - nightly
   - stable
-matrix:
-  allow_failures:
-    - rust: nightly
 addons:
   apt:
     sources:

src/memory.rs

@@ -110,7 +110,7 @@ impl MemoryInstance {
     /// [`LINEAR_MEMORY_PAGE_SIZE`]: constant.LINEAR_MEMORY_PAGE_SIZE.html
     pub fn alloc(initial: Pages, maximum: Option<Pages>) -> Result<MemoryRef, Error> {
         {
-            use std::convert::TryInto;
+            use core::convert::TryInto;
             let initial_u32: u32 = initial.0.try_into().map_err(|_| {
                 Error::Memory(format!("initial ({}) can't be coerced to u32", initial.0))
             })?;