Miscellanous fixes in preparation for 1.6.0

1. Bug fix in NumericDiffCostFunction (Thanks to Nicolas Brodu).
2. Minor documentation update in solver.h
3. Version history update.
4. Bump the version and ABI version.

Change-Id: I951574ddd0b2c4c03b9c79ff33eb9bea549071e7
diff --git a/include/ceres/ceres.h b/include/ceres/ceres.h
index ac76e97..b7aeeef 100644
--- a/include/ceres/ceres.h
+++ b/include/ceres/ceres.h
@@ -34,8 +34,8 @@
 #ifndef CERES_PUBLIC_CERES_H_
 #define CERES_PUBLIC_CERES_H_
 
-#define CERES_VERSION 1.5.0
-#define CERES_ABI_VERSION 1.5.0
+#define CERES_VERSION 1.6.0
+#define CERES_ABI_VERSION 1.6.0
 
 #include "ceres/autodiff_cost_function.h"
 #include "ceres/autodiff_local_parameterization.h"