remove deprecated use of warn_once (#217)

* remove deprecated use of warn_once

* update travis to test on 0.7

Author: mlubin

.travis.yml

@@ -4,7 +4,7 @@ os:
     - osx
 julia:
     - 0.6
-    - nightly
+    - 0.7
 notifications:
     email: false
 sudo: false

src/HighLevelInterface/HighLevelInterface.jl

@@ -6,7 +6,7 @@ using Compat
 
 function warn_no_inf(T)
     if !(isinf(typemin(T)) && isinf(typemax(T)))
-        Base.warn_once("Element type $T does not have an infinite value. Note that this may artifically introduce ranged (two-sided) constraints. To avoid this, consider casting the problem data to Float64.")
+        @Compat.warn("Element type $T does not have an infinite value. Note that this may artifically introduce ranged (two-sided) constraints. To avoid this, consider casting the problem data to Float64.")
     end
 end