update history

HISTORY.md

@@ -1,3 +1,10 @@
+## 1.7.3 (2015-07-25)
+
+* handle language standard target properties (e.g., `CXX_STANDARD`).
+* apply user provided prefix header to unity build target.
+* remove effect of `COTIRE_MINIMUM_NUMBER_OF_TARGET_SOURCES` on generated unity target.
+* manual updates.
+
 ## 1.7.2 (2015-04-22)
 
 * reduce configure time overhead.